This invention relates to a double-converting FM tuner.
Heretofore, a double-converting FM tuner has been constructed of an antenna, a radio frequency amplifier stage, a first frequency converter circuit composed of a first mixer circuit and a first local oscillator circuit, a first intermediate frequency filter, a first intermediate frequency amplifier stage, a second frequency converter circuit composed of a second mixer circuit and a second local oscillator circuit, a second intermediate frequency filter, and a demodulator circuit.
As in the case of the superheterodyne system, the double-converting system is prone to cause beat troubles. This is particularly true where an output signal of the first intermediate frequency amplifier stage is limit-amplified which results in the square pulse output current thereof containing large quantities of higher harmonic components. This leads to the disadvantage that the l-th order higher harmonics of the output current of the first intermediate frequency amplifier stage and the m-th order higher harmonics in the output current of the second local oscillator circuit or the n-th order higher harmonics of the second intermediate frequency give rise to cross modulation in the demodulator circuit. These difficulties appear at the output of the demodulator circuit. (Letters l, m and n denote positive integers.)
Usually, the second intermediate frequency filter operates normally at frequencies up to several tens of MHz. For frequencies from several tens of MHz to several hundred MHz, however, a sufficient decay is not attained in the rejection region of the filter on account of the characteristics of circuit components, etc. Accordingly, to inject the output current of the first intermediate frequency amplifier stage containing the large quantities of higher harmonic components directly into the second mixer circuit results in more higher harmonic components than are generated in the second frequency converter circuit when a first intermediate frequency signal not containing such higher harmonic components is supplied. The higher harmonic components increase the quantity of spurious waves to be mixed into the demodulator circuit so that the beat trouble is prone to occur. It has been revealed by the inventor that this phenomenon becomes especially conspicuous when the frequency converter circuits and the demodulator circuit are assembled within an identical semiconductor integrated circuit.